I. Field of the Invention
This invention relates generally to a gas analyzer apparatus, and more particularly to a gas analyzer of the cold-cathode glow-discharge type in which a gas sample introduced into an ionization chamber produces a glow whose intensity varies with a given gas concentration and whose spectrum varies with a given gas mixture.
II. Discussion of the Prior Art
In prior art gas analyzers of the so-called "Geisler tube" type, an unknown gas mixture is introduced into one end of a straight tube while a high vacuum is maintained within the tube by a suitable vacuum pump. When an appropriate high voltage is applied between the cathode and anode electrodes, ionization of the gas occurs, resulting in a glow and the emission of radiant energy in the infrared, visible and ultraviolet spectrum. By positioning a photodetector circuit adjacent the side wall of the spectrum-transparent tube, a signal is produced which, ideally, is proportional to the concentration of certain gases within the unknown mixture. Such prior art gas analyzers also utilize an optical filter disposed between the glow tube and the photodetector which is intended to pass a band of wavelengths unique to a particular gas. Thus, for example, if the device is to be used to measure the amount of nitrogen present in an air sample, it has been common practice to utilize a UV filter between the glow tube and the photodetector in that pure nitrogen classically exhibits a peak in the spectrum corresponding to UV wavelengths. This practice neglects the facts that: (1 ) a mixture of nitrogen and oxygen or carbon dioxide results in a shift in the peak radiation intensity toward the red wavelengths, and (2) that photodetectors exhibit poor sensitivity to ultraviolet and blue wavelengths.
The above-described prior art gas analyzer suffers from four major problems bearing upon its ability to provide consistent results over time. First of all, the walls of the tube tend to become clouded by impurities often introduced along with the sample being investigated or from the erosion of the electrodes caused by ion bombardment. This clouding results in a substantial decrease in sensitivity and premature replacement.
The second major defect centers around the fact that in a cold-cathode glow-discharge tube there is a variegated light pattern (banding). In particular, near the anode electrode a region called the "positive-column" develops. It is often also referred to as the plasma and consists of a series of rings separated by dark areas. Next to it is a region of low light intensity referred to as the "Faraday dark-space". As one moves toward the cathode, adjacent to the Faraday dark-space is the so-called "negative-glow" region, followed next by the cathode dark-space (Crookes dark-space) and finally the cathode glow region. It has been found that these ring-like regions do not remain stable but are subject to some movement or jitter within the tube, especially with the routinely encountered oscillatory changes in vacuum levels. Such jitter is a source of noise which necessarily detracts from the accuracy of the readings achievable using the device when the photodetector is oriented transverse to the direction of gas flow through the side wall of the glow-discharge tube.
The third limitation relates to the observation that nitrogen, in combination with oxygen, exhibits pronounced emissions in the orange and red spectra rather than in the ultraviolet. Conversely, major ultraviolet emissions appear most abundant with oxygen rather than nitrogen (the gas of interest).
Fourthly, commercial ultraviolet filters not only absorb most visible light, but also as much as 80% of the ultraviolet. When coupled with the typical insensitivity to ultraviolet of photodetectors, a very small portion of the actual emission is detected. This results in extremely poor signal-to-noise ratios.